The class of flutelike striation instabilities, including the Rayleigh-Taylor and E¿B gradient drift modes, acquire parallel variations as a result of the finite size and nonuniformity of the ionsphere. It is demonstrated that evanescence away from the source of free energy responsible for instability is the correct boundary condition that should be applied in the calculation of the eigenmodes for striations along the geomagnetic field. The appropriatences of the boundary condition arises because striation fields become more electrostatic at lower altitudes and cannot penetrate to the earth's surface. Ion-polarization currents and electromagnetic effects can limit the mapping of striation fields to the opposite conjugate zone, particularly when plasma densities are high and striations originate at higher geomagnetic latitudes.